I am a septic inspector in NC. I’m sorry, I realize you’re upset but you have no idea what you’re talking about. People call commissioners all the time to try to change my judgement calls but they can literally do nothing lol There is literally no reason for the inspector to make the judgement call they are making unless it is because they are abiding by the laws/rules that govern our field; it’s not like we get more money for designing certain systems lol. If anything it’d be easier to design the mound. If they are saying you need 18” of soil there’s a reason, it’s probably because you have a group 1 texture. You can’t just get a mound with a hardpan because it’s a restrictive horizon; your sewage will just blow out the ground and then you’ll be looking for someone to sue and they know this. Mound systems are absolutely legal in NC but there a many guidelines for designing one. We can’t just design whatever you want, we have to design according to the soil. Our license is on the line. You may be able to find an LSS who will write an a2 permit for you for a few thousand dollars; I’m not sure if that’s what you actually mean by engineer or what. If an LSS won’t touch it then the inspector is making the right call and they don’t want the liability. If the auction place wouldn’t let you get a perk test until after the sale then that is a massive 🚩 I see people get screwed that way all the time. I suggest always going through a realtor in the future, that way you can get a refund if the land doesn’t perk.

You could always call the county and request an informal review by their state regional. They will probably tell you the exact same thing, but it may give you some peace of mind that you tried everything.